Semiconductor device and forming method thereof

ABSTRACT

A semiconductor device includes a semiconductor substrate, at least one gate stack, a gate spacer and a dielectric cap. The gate stack is located on the semiconductor substrate. The gate spacer is located on a sidewall of the gate stack. The gate spacer includes a first dielectric layer and a second dielectric layer with different etch properties. The dielectric cap at least caps the gate spacer. The dielectric cap and the second dielectric layer define a gap therebetween.

BACKGROUND

In the race to improve transistor performance as well as reduce the sizeof transistors, transistors have been developed such that the channeland source/drain regions are located in a fin on a bulk substrate. Suchnon-planar devices can be referred to as multiple-gate finFETs. Amultiple-gate finFET may have a gate electrode that straddles across afin-like silicon body to form a channel region.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 to FIG. 15 are cross-sectional views of a method formanufacturing a semiconductor device at various stages in accordancewith some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Examples of devices that can be improved from one or more embodiments ofthe present disclosure are semiconductor devices. Such a device, forexample, is a Fin field effect transistor (FinFET) device. The followingdisclosure will continue with a FinFET example to illustrate variousembodiments. It is understood, however, that the disclosure is notlimited to a particular type of device.

FIG. 1 to FIG. 15 are cross-sectional views of a method formanufacturing a semiconductor device at various stages in accordancewith some embodiments of the present disclosure. Reference is made toFIG. 1. A semiconductor fin 110 is formed on the substrate 100 andprotrudes from the substrate 100. In some embodiments, the substrate 100includes silicon. Alternatively, the substrate 100 may includegermanium, silicon germanium, gallium arsenide or other appropriatesemiconductor materials. Also alternatively, the substrate 100 mayinclude an epitaxial layer. For example, the substrate 100 may have anepitaxial layer overlying a bulk semiconductor. Further, the substrate100 may be strained for performance enhancement. For example, theepitaxial layer may include a semiconductor material different from thatof the bulk semiconductor, such as a layer of silicon germaniumoverlying bulk silicon or a layer of silicon overlying bulk silicongermanium. Such strained substrate may be formed by selective epitaxialgrowth (SEG). Furthermore, the substrate 100 may include asemiconductor-on-insulator (SOI) structure. Also alternatively, thesubstrate 100 may include a buried dielectric layer, such as a buriedoxide (BOX) layer, such as that formed by separation by implantation ofoxygen (SIMOX) technology, wafer bonding, SEG, or other appropriatemethod.

In some embodiments, the semiconductor fin 110 includes silicon. Thesemiconductor fin 110 may be formed, for example, by patterning andetching the substrate 100 using photolithography techniques. In someembodiments, a layer of photoresist material (not shown) is sequentiallydeposited over the substrate 100. The layer of photoresist material isirradiated (exposed) in accordance with a desired pattern (thesemiconductor fin 110 in this case) and developed to remove portions ofthe photoresist material. The remaining photoresist material protectsthe underlying material from subsequent processing steps, such asetching. It is noted that other masks, such as an oxide or siliconnitride mask, may also be used in the etching process.

An isolation dielectric 120 is formed to fill trenches between thesemiconductor fins 110 as shallow trench isolation (STI). The isolationdielectric 120 may include any suitable dielectric material, such assilicon oxide. The method of forming the isolation dielectric 120 mayinclude depositing an isolation dielectric 120 on the substrate 100 tocover the semiconductor fin 110, optionally performing a planarizationprocess, such as a chemical mechanical polishing (CMP) process, toremove the excess isolation dielectric 120 outside the trenches, andthen performing an etching process on the isolation dielectric 120 untilupper portions of the semiconductor fins 110 are exposed. In someembodiments, the etching process performed may be a wet etching process,for example, by dipping the substrate 100 in hydrofluoric acid (HF). Inalternative embodiments, the etching process may be a dry etchingprocess, for example, the dry etching process may be performed usingCHF₃ or BF₃ as etching gases.

Reference is made to FIG. 2. Gate stacks 130 are formed on portions ofthe semiconductor fin 110 at interval and expose other portions of thesemiconductor fin 110. In some embodiments using a gate-last process,the gate stacks 130 are dummy gates and at least portions thereof willbe replaced by final gate stacks at a subsequent stage. For example,portions of the dummy gate stacks 130 are to be replaced later by metalgate electrodes (MG) after high temperature thermal processes, such asthermal annealing for source/drain activation during the sources/drainsformation. In some embodiments, the dummy gate stacks 130 include gatedielectrics 132, dummy electrodes 134 and gate masks 136. In someembodiments, the gate dielectrics 132 may include, for example, a high-kdielectric material such as metal oxides, metal nitrides, metalsilicates, transition metal-oxides, transition metal-nitrides,transition metal-silicates, oxynitrides of metals, metal aluminates,zirconium silicate, zirconium aluminate, or combinations thereof. Insome embodiments, the gate dielectrics 132 may include hafnium oxide(HfO₂), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride(HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HMO),hafnium zirconium oxide (HfZrO), lanthanum oxide (LaO), zirconium oxide(ZrO), titanium oxide (TiO), tantalum oxide (Ta₂O₅), yttrium oxide(Y₂O₃), strontium titanium oxide (SrTiO₃, STO), barium titanium oxide(BaTiO₃, BTO), barium zirconium oxide (BaZrO), hafnium lanthanum oxide(HfLaO), lanthanum silicon oxide (LaSiO), aluminum silicon oxide(AlSiO), aluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), oxynitrides(SiON), and combinations thereof. In alternative embodiments, the gatedielectrics 132 may have a multilayer structure such as one layer ofsilicon oxide (e.g., interfacial layer) and another layer of high-kmaterial. The dummy electrodes 134 may include polycrystalline silicon(polysilicon), as examples. The gate masks 136 may include a suitabledielectric material, such as silicon nitride, silicon oxynitride orsilicon carbide, as examples.

The dummy gate stacks 130 can be formed by deposition and patterning.For example, the gate dielectric 132 is blanket deposited on thestructure shown in FIG. 1 by a suitable technique, such as chemicalvapor deposition (CVD). The dummy electrode 134 is deposited on the gatedielectric 132 by a suitable technique, such as CVD. The gate mask 136is deposited on the dummy electrode 134 by a suitable technique, such asCVD. Then the gate mask 136 is patterned by a lithography process and anetching process, thereby forming openings in the gate mask 136, exposingthe underlying dummy gate materials within the openings. The lithographyprocess may include photoresist (or resist) coating (e.g., spin-oncoating), soft baking, mask aligning, exposure, post-exposure baking,developing the photoresist, rinsing, drying (e.g., hard baking), othersuitable processes, and/or combinations thereof. The etching processincludes dry etching, wet etching, and/or other etching methods (e.g.,reactive ion etching). Another etching process is applied to the dummygate materials through the openings of the gate mask 136 using the gatemask 136 as an etch mask, thereby forming the gate stacks 130 straddlingportions of the semiconductor fin 110.

Reference is made to FIG. 3. A blanket first dielectric layer 140 isformed on the structure shown in FIG. 2. That is, the first dielectriclayer 140 is conformally formed over at least the semiconductor fin 110and the dummy gate stacks 130. In some embodiments, the first dielectriclayer 140 may include silicon oxide, silicon nitride, siliconoxy-nitride, or other suitable material. In some embodiments, the firstdielectric layer 140 includes non-porous dielectric materials, which maybe advantageous to resist against subsequent etching processes, such asetching in a gate replacement process. The first dielectric layer 140may be formed by a deposition process, such as an atomic layerdeposition (ALD) process, a CVD process, a physical vapor deposition(PVD) process, a sputter deposition process or other suitabletechniques.

Next, a second dielectric layer 150 is formed on the first dielectriclayer 140, and the second dielectric layer 150 is conformal to the firstdielectric layer 140. The second dielectric layer 150 has a dielectricconstant less than that of the first dielectric layer 140. For example,the second dielectric layer 150 may include a low-k dielectric materialhaving a dielectric constant less than about 4.0. In some embodiments,the dielectric constant of the second dielectric layer 150 may rangefrom about 3.0 to about 3.9, and the dielectric constant of the firstdielectric layer 140 may be greater than about 4.0, such as in a rangefrom about 4.5 to about 6.5. The second dielectric layer 150 includingthe low-k dielectric material may be deposited using ALD, PVD or a CVDmethod such as plasma enhanced CVD (PECVD), low pressure CVD (LPCVD), oratomic layer CVD (ALCVD). In some embodiments, the second dielectriclayer 150 may include porous low-k carbon-containing materials such as,for example, porous silicon oxycarbonitride (SiOCN), porous siliconoxycarbide (SiOC), porous silicon carbide (SiC), or other suitableporous dielectric materials. The porous low-k dielectric materials maybe beneficial to reduce a parasitic capacitance between a metal gatestack and a contact plug formed in subsequently steps due to itsporosity, and a resistive-capacitive (RC) time delay caused by theparasitic capacitance can be thus decreased. Moreover, the porous low-kdielectric layer 150 and the first dielectric layer 140 have differentetch properties. For example, the first and second dielectric layers 140and 150 have different etch resistance properties. That is, the firstdielectric layer 140 is made of a material which has higher etchresistance to an etchant used to etch the second dielectric layer 150.Therefore, a portion of the porous low-k dielectric layer 150 can beselectively etched to create a gap adjacent to the first dielectriclayer 140 in a subsequently process, and the gap may have an extremelylow dielectric constant due to gas, especially air, contained therein.The parasitic capacitance can be further reduced, and the RC time delaycan be further decreased, accordingly.

In some embodiments, the second dielectric layer 150 may include otherlow-k dielectric materials, such as carbon doped silicon dioxide, low-ksilicon nitride, low-k silicon oxynitride, polyimide, spin-on glass(SOG), fluoride-doped silicate glass (FSG), phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), Black Diamond® (Applied Materials ofSanta Clara, Calif.), Xerogel, Aerogel, amorphous fluorinated carbon,Parylene, BCB (bis-benzocyclobutenes), SiLK (Dow Chemical of Midland,Mich.), other suitable low-k dielectric materials, and/or combinationsthereof.

Reference is made to FIG. 4. A removal process is performed to removeportions of the first and second dielectric layers 140 and 150, andremaining portions of the first and second dielectric layers 140 and 150can collectively serve as gate spacers 160 located on opposite sides ofthe dummy gate stacks 130. That is, two gate spacers 160 arerespectively located on two opposite sidewalls 131 of a dummy gate stack130. The removal process may be, for example, an etching process, suchas an anisotropic etching process. In some embodiments, the gate spacer160 has the non-porous first dielectric layer 140 located between theporous second dielectric layer 150 and the dummy gate stack 130, andtherefore, the gate spacer 160 can be capable of resisting against theetching in the gate replacement process. In some embodiments, the gatespacers 160 may be used to offset subsequently formed epitaxy structureson the semiconductor fin 110, such as source/drain epitaxy structures.The gate spacers 160 may further be used for designing or modifyingsource/drain regions (junction) profile.

Reference is made to FIG. 5. Portions of the semiconductor fin 110exposed by the dummy gate stacks 130 and the gate spacers 160 areremoved (or recessed) to form recesses R in the semiconductor fin 110.Any suitable amount of material may be removed. The remainingsemiconductor fin 110 has a plurality of source/drain portions 110 s anda channel portion 110 c between the source/drain portions 110 s.Portions of the source/drain portions 110 s are exposed by the recessesR. The channel portions 110 c respectively underlie the dummy gatestacks 130.

Removing portions of the semiconductor fin 110 may include forming aphotoresist layer or a capping layer (such as an oxide capping layer)over the structure of FIG. 4, patterning the photoresist or cappinglayer to have openings that expose portions of the semiconductor fin110, and etching the exposed portions of the semiconductor fin 110. Insome embodiments, the semiconductor fin 110 can be etched using a dryetching process. Alternatively, the etching process is a wet etchingprocess, or combination dry and wet etching process. Removal may includea lithography process to facilitate the etching process. The lithographyprocess may include photoresist coating (e.g., spin-on coating), softbaking, mask aligning, exposure, post-exposure baking, developing thephotoresist, rinsing, drying (e.g., hard baking), other suitableprocesses, or combinations thereof. Alternatively, the lithographyprocess is implemented or replaced by other methods, such as masklessphotolithography, electron-beam writing, and ion-beam writing. In yetsome other embodiments, the lithography process could implementnanoimprint technology. In some embodiments, a pre-cleaning process maybe performed to clean the recesses R and with HF or other suitablesolution.

Reference is made to FIG. 6. A plurality of epitaxy structures 170 arerespectively formed in the recesses R and on the source/drain portions110 s of the semiconductor fin 110. The epitaxy structures 170 may beformed using one or more epitaxy or epitaxial (epi) processes, such thatSi features, SiGe features, and/or other suitable features can be formedin a crystalline state on the source/drain portions 110 s of thesemiconductor fin 110. In some embodiments, the lattice constants of theepitaxy structures 170 are different from the lattice constant of thesemiconductor fin 110, so that the channel portions 110 c of thesemiconductor fin 110 can be strained or stressed by the epitaxystructures 170 to improve carrier mobility of the semiconductor deviceand enhance the device performance. The epitaxy processes include CVDdeposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-highvacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or other suitableprocesses. The epitaxy process may use gaseous and/or liquid precursors,which interact with the composition of the source/drain portions 110 sof the semiconductor fin 110 (e.g., silicon). Thus, a strained channelcan be achieved to increase carrier mobility and enhance deviceperformance.

Reference is made to FIG. 7. A screening layer 180 is blanket formed onthe structure shown in FIG. 6. That is, the screening layer 180 caps thesemiconductor fin 110, the dummy gate stacks 130, the gate spacers 160and the epitaxy structures 170. The screening layer 180 may be used forimplantation screening and reduction of the channeling effect during thesubsequent implantation. The screening layer 180 may be an oxide layer,ranging from about 10 angstroms to about 50 angstroms in thickness.Formation of the screening oxide layer 180 may exemplarily includedeposition, such as PVD or CVD.

Reference is made to FIG. 8. A plasma treatment P is performed to thesecond dielectric layers 150 of the gate spacers 160. The plasmatreatment P can cause depletion of carbon containing moieties within thesurface region of the second dielectric layer 150. The depletion ofcarbon in the second dielectric layers 150 may be advantageous toimprove a wet etching rate in its etchant, such as the dilutehydrofluoric acid (DHF). The plasma treatment P may include, forexample, exposing the structure shown in FIG. 8 to a plasma environment,such as an oxygen plasma environment.

Next, an implantation process is performed to implant dopant into theepitaxy structures 170. The doping species may include p-type dopant,such as boron or BF₂; n-type dopant, such as phosphorus or arsenic;and/or other suitable dopant including combinations thereof. One or moreannealing processes may be performed to activate the epitaxy structures170. The annealing processes include rapid thermal annealing (RTA)and/or laser annealing processes.

Reference is made to FIG. 9. The screening layer 180 shown in FIG. 8 isremoved using, for example, an etching process, and a blanket thirddielectric layer 190 is then formed on substrate 100. That is, the thirddielectric layer 190 is formed on the semiconductor fin 110, the dummygate stacks 130, the gate spacers 160 and the epitaxy structures 170.The third dielectric layer 190 may serve as an etch stop layer,especially a contact etch stop layer (CESL). The CESL may be made ofsilicon nitride, silicon oxynitride or other suitable materials. Thethird dielectric layer 190 may be formed by plasma enhanced CVD, lowpressure CVD, ALD, or other applicable processes. In some embodiments,the second and third dielectric layers 150 and 190 include differentdielectric materials with different etch properties. More particularly,the second and third dielectric layers 150 and 190 have different etchresistance properties. That is, the third dielectric layer 190 is madeof a material which has higher etch resistance to the etchant used toetch the second dielectric layer 150. For example, the second dielectriclayer 150 may include a porous dielectric material, and the thirddielectric layer 190 may include a non-porous dielectric material.

Thereafter, as shown in FIG. 10, an interlayer dielectric (ILD) layer200 is formed on the substrate 100 the cover the semiconductor fin 110,the dummy gate stacks 130, the gate spacers 160 and the epitaxystructures 170. A portion of the ILD layer 200 may be formed between thedummy gate stacks 130 and may fill the remaining space between the dummygate stacks 130, and this portion of the ILD layer 200 can be referredto as a filling dielectric 202 located between the dummy gate stacks130. The ILD layer 200 includes silicon oxide, silicon nitride, siliconoxynitride, silicon carbide, low-k dielectric material or a combinationthereof. The ILD layer 200 includes a single layer or multiple layers.The ILD layer 200 is formed by a suitable technique, such as CVD.Afterward, a chemical mechanical polishing (CMP) process may be appliedto remove excessive ILD layer 200 and expose top surfaces of the dummygate stacks 130 to a subsequent dummy gate removal process. Moreover,this CMP process also exposes tops of the first, second and thirddielectric layers 140, 150, 190 and the ILD layer 200.

Reference is made to FIG. 11. At least portions of the dummy gate stacks130 (see FIG. 10) are removed to form openings O with the gate spacers160 as their sidewalls. In some embodiments, the dummy electrodes 134and the gate masks 136 are removed while the gate dielectrics 132 retainas shown in FIG. 11. In the embodiments where the gate dielectrics 132include high-k dielectric materials, the high-k gate dielectrics 132 areformed prior to the formation of the gate spacers 160, so that innerwalls of the gate spacers 160 may not be blanket covered by high-kdielectric materials. This arrangement may be beneficial to reduce theparasitic capacitance between the subsequently formed gate stack andcontact plug. Alternatively, in some other embodiments, the gatedielectrics 132 can be removed as well. The dummy gate stacks 130 may beremoved by dry etching, wet etching, or a combination of dry and wetetching. For example, a wet etching process may include exposure to ahydroxide containing solution (e.g., ammonium hydroxide), deionizedwater, and/or other suitable etchant solutions. In some embodiments, thenon-porous first dielectric layer 140 has higher etch resistance to theetching the dummy gate stacks 130 than that of the porous seconddielectric layer 150, and therefore, the non-porous first dielectriclayer 140 can protect the gate spacers 160 against the etching theopenings O.

Reference is made to FIG. 12. Gate conductors 214 are respectivelyformed in the openings O between the gate spacers 160. The gateconductors 214 and the respective underlying gate dielectrics 132 can becollectively referred to as gate stacks 210. The gate stacks 210straddle the semiconductor fin 110 and extend along the gate spacers160. The gate spacers 160 are present on sidewalls 211 of the gatestacks 210. In some embodiments, the gate conductors 214 may includework function metals to provide suitable work functions for the gatestacks 210. For example, if a P-type work function metal (P-metal) for aPMOS device is desired, P-type work function materials may be used.Examples of P-type work function materials include, but are not limitedto, titanium nitride (TiN), tungsten nitride (WN), tungsten (W),ruthenium (Ru), palladium (Pd), platinum (Pt), cobalt (Co), nickel (Ni),conductive metal oxides, and/or other suitable materials. On the otherhand, if an N-type work function metal (N-metal) for NMOS devices isdesired, N-type metal materials may be used. Examples of N-type workfunction materials include, but are not limited to, titanium aluminide(TiAl), titanium aluminum nitride (TiAlN), carbo-nitride tantalum(TaCN), hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta),aluminum (Al), metal carbides (e.g., hafnium carbide (HfC), zirconiumcarbide (ZrC), titanium carbide (TiC), aluminum carbide (AlC)),aluminides, and/or other suitable materials. The gate conductors 214 mayfurther include filling metals located on the work function metals andfilling recesses in the work function metals. The filling metals mayinclude tungsten, aluminum, copper, nickel, cobalt, titanium, tantalum,titanium nitride, tantalum nitride, nickel silicide, cobalt silicide,TaC, TaSiN, TaCN, TiAl, TiAlN, or other suitable materials.

Exemplary method of forming the gate conductors 214 may include blanketforming one or more work function metal layers over the structure shownin FIG. 11, forming filling metal over the work function metal layers,wherein some portions of the filling metal overfill the openings O shownin FIG. 11, and then performing a CMP process to remove excessivefilling metal and work function metal layers outside the openings O.

Reference is made to FIG. 13. At least a portion of the seconddielectric layer 150 is removed. In particular, a top portion of thesecond dielectric layer 150 is removed, while underlying portions of thesecond dielectric layer 150 remain between the first dielectric layer140 and the third dielectric layer 190, and the remaining portions ofthe second dielectric layer 150 can be referred to as a lowereddielectric layer 155. This removal process creates a gap G between thefirst and third dielectric layers 140 and 190. That is, sidewalls 142and 192 of the first and third dielectric layers 140 and 190 are exposeddue to the removal process. In other words, the removing the top portionof the second dielectric layer 150 leaves a gap G overlying the lowereddielectric layer 155, and the first and third dielectric layers 140 and190 are separated at least by the gap G. That is, the gap G is locatedon a top of the lowered dielectric layer 155, or stated differently, thelowered dielectric layer 155 is located on a bottom of the gap G. Thegap G may be a gas-containing or gas-filled space, especially anair-containing or air-filled space, and the gap G can thus be referredto as an air gap. Gas in the gap G may separate the first dielectriclayer 140 and the third dielectric layer 190, and therefore, the gas,especially air, can provide a dielectric feature with extremely lowdielectric constant (about 1) between the first and third dielectriclayers 140 and 190, so that the parasitic capacitance between the gatestack 210 and a subsequently formed contact plug can be further reduced,and the RC time delay can be further decreased.

The etch property difference among the second dielectric layer 150 andthe first and third dielectric layers 140 and 190 may benefit theformation of the gap G between the first and third dielectric layers 140and 190. For example, the formation of the gap G may include an etchingprocess, such as a wet etching process, a dry etching process orcombinations thereof. During the etching process, an etch selectivity ofthe second dielectric layer 150 is higher than that of the first andthird dielectric layers 140 and 190. Stated differently, the first andthird dielectric layers 140 and 190 have higher etch resistances to theetchant used to etch the second dielectric layer 150. Accordingly, thefirst and third dielectric layers 140 and 190 are not easier to beetched or removed compared to the second dielectric layer 150 duringthis etching process. Therefore, a height difference between the lowereddielectric layer 155 and the first and third dielectric layers 140 and190 are generated after the etching process, and the gap G is thusformed between the first and third dielectric layers 140 and 190 andatop the lowered dielectric layer 155. In some embodiments, the etchantused in this etching process may be dilute hydrofluoric acid (DHF), HF,or other suitable materials.

In some embodiments, the plasma treatment P performed in FIG. 8 causesdepletion of carbon in the second dielectric layer 150, and thisdepletion of carbon may be advantageous to improve the wet etching rateof the second dielectric layer 150 in the DHF. The improved wet etchingrate may be beneficial to create a deeper gap G between the first andthird dielectric layers 140 and 190. The deeper the gap G is, the morethe contained gas or air is. As such, the reduction of the parasiticcapacitance between the gate stack 210 and the subsequently contact plugcan be further enhanced due to the plasma treatment P.

In some embodiments, the removal process performed to the seconddielectric layer 150 also removes a portion of the ILD layer 200, sothat the ILD layer 200 has a top lower than that of the first and thirddielectric layers 140 and 190. In other words, the filling dielectric202 between the gate stacks 210 has a top lower than that of the firstand third dielectric layers 140 and 190. In some embodiments, theremoval process performed to the second dielectric layer 150 may alsoremove some portions of the first and third dielectric layers 140 and190. The etch resistances of the first and third dielectric layers 140and 190 to the etchant used to etch the second dielectric layer 150 arehigher than that of the ILD layer 200. This etch resistance differencemakes height loss of the ILD layer 200 greater than height losses of thefirst and third dielectric layers 140 and 190, and therefore, the ILDlayer 200 can have the top lower than that of the first and thirddielectric layers 140 and 190. In some embodiments, the ILD layer 200has higher etch resistance to the etchant used to etch the seconddielectric layer 150, and therefore, the height loss of the ILD layer200 is less than that of the second dielectric layer 150. That is, thetop of the ILD layer 200 is higher than that of the lowered dielectriclayer 155. In some embodiments, ratio of the height loss of the ILDlayer 200 to the height loss of the second dielectric layer 150 may beabout 1:5. In some embodiments, the height loss of the second dielectriclayer 150 may range from about 120 angstroms to about 180 angstroms.That is, height of the gap G may range from about 120 angstroms to about180 angstroms. In some embodiments, the height loss of the ILD layer 200may range from about 10 angstroms to about 50 angstroms. In someembodiments, portions of the gate stacks 210 may be removed due to theetching process performed to the second dielectric layer 150. The gatestacks 210 have higher etch resistance to the etchant used to etch thesecond dielectric layer 150, so that tops of the gate stacks 210 arehigher than that of the lowered dielectric layer 155. In someembodiments, height losses of the gate stacks 210 due to this etchingprocess may be less than about 10 angstroms.

Reference is made to FIG. 14. A dielectric cap 220 is formed on thestructure shown in FIG. 13. The dielectric cap 220 at least caps thegate spacers 160, and the gap G is located between the dielectric cap220 and the lowered dielectric layer 155. That is, sidewalls 142 and 192of the first and third dielectric layers 140 and 190 are not covered bythe dielectric cap 220. In some embodiments, the dielectric cap 220further caps the third dielectric layer 190, the gate stacks 210 and theILD layer 200. The gap G can be thus sealed at least by the overlyingdielectric cap 220, and gas in the gap G may separate the lowereddielectric layer 155 and the dielectric cap 220. In some embodiments,the dielectric cap 220 and the lowered dielectric layer 155 arerespectively located on two opposite sides of the gap G, and the thirddielectric layer 190 and the first dielectric layer 140 are respectivelylocated on another two opposite sides, and the gap G can therefore bedefined by the first and third dielectric layers 140 and 190, thelowered dielectric layer 155 and the dielectric cap 220. Stateddifferently, sidewalls 142 and 192 of the first and third dielectriclayers 140 and 190, the top of the lowered dielectric layer 155, and aportion of a bottom of the dielectric cap 220 are exposed to the gap G.The dielectric cap 220 is advantageous to keep gas or air in the gap G.That is, the dielectric cap 220 is advantageous to prevent the gap Gfrom being filled by unwanted materials. As such, the extremely low kdielectric feature formed by the gap G can be remained in the finalproduct to reduce the parasitic capacitance. In some embodiments, adistance between the first and third dielectric layers 140 and 190 isshort enough to prevent the dielectric cap 220 from filling the gap G.Stated differently, the gap G is narrow enough to prevent the dielectriccap 220 from filling the gap G after deposition of the dielectric cap220. In some embodiments, the dielectric cap 220 has a thickness rangingfrom about 10 angstroms to about 50 angstroms. Exemplary method offorming the dielectric cap 220 may include CVD, PVD, or other suitabletechniques.

In some embodiments, a gap may be absent between the top of the firstdielectric layer 140 and the dielectric cap 220. For example, the top ofthe first dielectric layer 140 may be in contact with the dielectric cap220. Similarly, a gap may be absent between the dielectric cap 220 andtops of the third dielectric layer 190, the ILD layer 200 and the gatestacks 210. That is, the tops of the third dielectric layer 190, the ILDlayer 200 and the gate stacks 210 may be in contact with the dielectriccap 220. In other words, the gap G does not extend to between thedielectric cap 220 and the tops of the first and third dielectric layers140 and 190, the ILD layer 200 and the gate stacks 210.

Next, another interlayer dielectric (ILD) layer 230 is formed on thedielectric cap 220. The ILD layer 230 includes silicon oxide, siliconnitride, silicon oxynitride, silicon carbide, low-k dielectric materialor a combination thereof. The ILD layer 230 includes a single layer ormultiple layers. The ILD layer 230 is formed by a suitable technique,such as CVD.

Reference is made to FIG. 15. Conductive features, such as contact plugs240, are formed through the ILD layers 230 and 200, the dielectric cap220 and the third dielectric layer 190, and they are in contact withtops of the epitaxy structures 170, respectively. The contact plugs 240can thus serve as source/drain contacts. The gap G is located betweenthe contact plug 240 and the gate stack 210, and therefore, the gap Gcan reduce the parasitic capacitance between the gate stack 210 and thecontact plug 240, thereby decreasing the RC time delay. Further, thelowered dielectric layer 155 has the dielectric constant lower than thefirst dielectric layer 140, and the lowered dielectric layer 155 islocated between the gate stack 210 and the contact plug 240. Therefore,the lowered dielectric layer 155 is also beneficial to reduce theparasitic capacitance between the gate stack 210 and the contact plug240, thereby decreasing the RC time delay.

Exemplary formation method of the contact plugs 240 may include formingcontact holes by one or more etching processes to sequentially etchthrough the ILD layer 230, the dielectric cap 220 and the ILD layer 200down to the respective epitaxy structures 170, and depositing metal orother suitable conductive materials in the contact holes by a depositionprocess, such as a CVD process, to form the contact plugs 240.

In some embodiments, there is a gap between the contact plug and thegate stack of the semiconductor device. The gap is advantageous toreduce the parasitic capacitance between the gate stack and the contactplug. The RC time delay can be decreased, accordingly. Moreover, the gapis located on a lowered dielectric layer having an etch propertydifferent from that of a dielectric layer adjacent to the gate stack.Therefore, the gap can be formed by an etching process, in which thedielectric layer adjacent to the gate stack has higher etch resistanceto the etchant to etch the lowered dielectric layer. Further, thelowered dielectric layer has lower dielectric constant compared to thedielectric layer adjacent to the gate stack, so the parasiticcapacitance between the gate stack and the contact plug can be furtherreduced.

According to some embodiments, a semiconductor device includes asemiconductor substrate, at least one gate stack, a gate spacer and adielectric cap. The gate stack is located on the semiconductorsubstrate. The gate spacer is located on a sidewall of the gate stack.The gate spacer includes a first dielectric layer and a seconddielectric layer with different etch properties. The dielectric cap atleast caps the gate spacer. The dielectric cap and the second dielectriclayer define a gap therebetween.

According to some embodiments, a semiconductor device includes asemiconductor substrate, at least one gate stack, a gate spacer and athird dielectric layer. The gate stack is located on the semiconductorsubstrate. The gate spacer is located on a sidewall of the gate stack.The gate spacer includes a first dielectric layer and a seconddielectric layer. The third dielectric layer is located on thesemiconductor substrate. The third dielectric layer and the firstdielectric layer are separated at least by a gap. The second dielectriclayer is located on a bottom of the gap.

According to some embodiments, a method of forming a semiconductordevice includes forming a gate spacer on a semiconductor substrate, thegate spacer including a first dielectric layer and a second dielectriclayer located on the first dielectric layer; forming a gate stackadjacent to the first dielectric layer of the gate spacer; removing atleast a portion of the second dielectric layer such that a sidewall ofthe first dielectric layer is exposed; and forming a dielectric cap onthe gate spacer, wherein at least a portion of the sidewall of the firstdielectric layer is not covered dielectric cap.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate; a gate stack on the semiconductor substrate; agate spacer on a sidewall of the gate stack, the gate spacer comprisinga first dielectric layer and a second dielectric layer that has adielectric constant less than that of the first dielectric layer, thefirst dielectric layer located between the second dielectric layer andthe gate stack; a third dielectric layer surrounding the first andsecond dielectric layers; and a dielectric cap capping the first,second, and third dielectric layers, wherein the dielectric cap and thefirst, second, and third dielectric layers define an air gaptherebetween, the first dielectric layer is further located between thesecond dielectric layer and the semiconductor substrate and extends tothe dielectric cap, and the air gap is located on a top of the seconddielectric layer.
 2. The semiconductor device of claim 1, wherein theair gap separates the second dielectric layer and the dielectric cap. 3.The semiconductor device of claim 1, wherein the top of the seconddielectric layer is lower than that of the first dielectric layer. 4.The semiconductor device of claim 1, further comprising: a conductivefeature on the semiconductor substrate, the air gap being locatedbetween the conductive feature and the gate stack.
 5. The semiconductordevice of claim 1, further comprising: a conductive feature on thesemiconductor substrate, the second dielectric layer being locatedbetween the conductive feature and the gate stack.
 6. The semiconductordevice of claim 1, wherein the second dielectric layer is a porousdielectric layer.
 7. The semiconductor device of claim 1, furthercomprising: a filling dielectric surrounding the gate stack and having atop lower than that of the first dielectric layer.
 8. The semiconductordevice of claim 7, wherein the top of the filling dielectric is higherthan that of the second dielectric layer.
 9. The semiconductor device ofclaim 1, wherein the dielectric cap is in contact with oppositesidewalls of the third dielectric layer.
 10. A semiconductor device,comprising: a semiconductor substrate; a gate stack on the semiconductorsubstrate; a gate spacer on a sidewall of the gate stack, the gatespacer comprising a first dielectric layer and a second dielectriclayer; an epitaxy structure on the side of the gate stack; and a thirddielectric layer on the semiconductor substrate, the third dielectriclayer and the first dielectric layer being separated by an air gap, thesecond dielectric layer being in contact with a sidewall of the epitaxystructure and located between a bottom of the air gap and the firstdielectric layer, and the third dielectric layer being in contact with atop surface of the epitaxy structure.
 11. The semiconductor device ofclaim 10, wherein the air gap has gas therein, and the gas separates thefirst dielectric layer and the third dielectric layer.
 12. Thesemiconductor device of claim 10, wherein the first dielectric layer isfurther located between the air gap and the gate stack.
 13. Thesemiconductor device of claim 10, wherein the first dielectric layer isin contact with the sidewall of the epitaxy structure.
 14. Thesemiconductor device of claim 10, wherein a topmost surface of thesecond dielectric layer is lower than a top surface of the gate stack.15. A semiconductor device comprising: a substrate; a gate stack abovethe substrate; and a gate spacer on a sidewall of the gate stack andincluding first, second, and third dielectric layers, wherein the seconddielectric layer is between the first and third dielectric layers, asidewall of the first dielectric layer, a sidewall of the thirddielectric layer, and a topmost surface of the second dielectric layerdefine an air gap and a bottom surface of the third dielectric layer isover a bottom surface of the first dielectric layer, and an epitaxystructure above the substrate, wherein the second dielectric layer is incontact with the epitaxy structure.
 16. The semiconductor device ofclaim 15, further comprising an epitaxial structure over the substrate,wherein the first dielectric layer is on a sidewall of the epitaxialstructure.
 17. The semiconductor device of claim 15, further comprisingan epitaxial structure over the substrate, wherein the second dielectriclayer is on a sidewall of the epitaxial structure.
 18. The device ofclaim 15, further comprising an epitaxial structure over the substrate,wherein the third dielectric layer is on opposite sidewalls of theepitaxial structure.
 19. The device of claim 15, further comprising afin above the substrate, wherein the second dielectric layer is on asidewall of the fin.
 20. The semiconductor device of claim 15, furthercomprising a dielectric cap in contact with the gate stack and the firstand third dielectric layers.